Fix pre-commit mypy error:

```
mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

ubench/holder_comparison.py:12: error: Unused "type: ignore" comment, use narrower [import-not-found] instead of [import] code  [unused-ignore]
```

ubench/holder_comparison.py

@@ -9,7 +9,7 @@
 import time
 from typing import Any, Callable, Dict, List
 
-import pybind11_ubench_holder_comparison as m  # type: ignore[import]
+import pybind11_ubench_holder_comparison as m  # type: ignore[import-not-found]
 
 number_bucket_pc = None